The present invention relates generally to power measuring apparatus and deals more particularly with an improved apparatus or circuit for producing an output signal proportional to the product of two input signals.
Generally the power supplied to an electrical device such as, for example, a home appliance, can be determined from the resultant product of the magnitudes of the voltage measured across the input terminals to the device and the current supplied to the device. Power measuring apparatus, such as power meters, power monitors and the like are often designed to operate with and compute power from voltage and current signals having sinusoidal waveshapes and magnitudes within a predetermined range. Power measuring apparatus of the aforementioned type generally have a limited dynamic measuring range and require sinusoidal waveshape signals for proper operation. However, voltage and current waveshapes are not always sinusoidal and may often exceed anticipated maximum values. It is useful and desirable in many instances to know the mean value of instantaneous power supplied to a device and therefore the measuring apparatus must be capable of operating with instantaneous voltages and currents varying over several orders of magnitude.
In order to more accurately measure power for voltage and current magnitudes outside a preselected range, a different power measuring range corresponding to the magnitudes of the particular range of voltage and current associated with the electrical device being measured must be selected, which selection conditions the measuring apparatus to operate in the selected measuring range. For example, a power range selected in a measuring apparatus having an accuracy of 1% full scale reading would read within .+-.20 watts of the actual power reading when the 2,000 watt range is selected; that is, a power of 2,000 watts might be indicated in the range of 1,980 to 2,020 watts and a power of 200 watts might be indicated in the range of 180 to 220 watts. It is readily seen that the 200 watt reading using the 2,000 watt selected range is subject to an approximate error of 10%. Therefore, it is necessary to select a lower maximum measuring range, say 200 watts, to measure with an accuracy of .+-.2 watts. It will be seen that a measured power of 2 watts might not even be read when measured with a 200 watt selected range. Consequently, either the user or the power measuring apparatus itself must select an appropriate measuring range to obtain a reasonably accurate power measurement at a full scale reading.
Changing the selected measuring range to accommodate fluctuating input voltage and current signals or varying operating conditions of the electrical device connected to the measuring apparatus is often inconvenient because the user may be making repairs, adjustments and the like to the device while observing changes in the measured power and the changes may cover one or more ranges. Auto-ranging power measuring apparatus is generally very costly and possesses accuracy characteristics similar to manually range selected type power measuring apparatus.
One problem often associated with the lack of accuracy over a wide measuring range in a low cost power measuring apparatus is the presence of a DC offset voltage in the power output signal. The output signal is generally produced by a multiplier circuit of some type which computes the product of the voltage and current supplied to the electrical device connected to the measuring apparatus.
The voltage-current product computation can be implemented using a standard commercially available four-quadrant analog multiplier integrated circuit however, power measuring apparatus implemented using such integrated circuits generally exhibit an output error due to the pesence of a DC offset voltage in the output signal and which error is generally in the range of 1% of full scale for the measuring range selected. In addition, integrated circuit multipliers are generally costly and require several external components to function properly. Other, more conventional, multiplier circuits implemented using low cost, discrete components also exhibit static output errors in the range of 1% of full scale due to DC offset voltage in the output signal caused in part by circuit components having unmatched characteristics and in part by the design of the multiplier. The DC offset voltage can be reduced somewhat by utilizing circuit compensating techniques such as null suppression and balancing. Generally such compensation also requires additional circuit components and adds to the cost and complexity of the multiplier circuit and the power measuring apparatus.
The purpose of the invention is therefore to provide a low cost easily implemented multiplier circuit for producing an output voltage signal proportional to the product of two unknown input voltage signals particularly useful in implementing a low cost power measuring apparatus.
Another aim of the present invention is to provide a digital readout power measuring apparatus constructed using low cost, standard solid-state components and having a wide dynamic measuring range in the order of 200 to 1.
It is yet a further aim of the present invention to provide a multiplier circuit for producing a DC output voltage proportional to the product of two input voltage signals which DC output voltage signal has substantially zero DC offset voltage to produce a static output error of substantially less than 0.05% of the full scale reading.
Other objects and advantages of the invention will be apparent from the following description and claims taken in conjunction with the accompanying drawings.